Field of the Invention
The present invention relates in general to the field of flight control of aircraft. In particular, the present invention relates to apparatus and methods for controlling the flight of a tiltrotor aircraft.
Description of Related Art
A rotary wing aircraft, such as a helicopter or the tiltrotor aircraft 11 shown in FIG. 1, produces lift with at least one main rotor 13, which comprises multiple wings, or blades 15, attached to a rotating hub 17. Each blade 15 has an airfoil cross-section, and lift is produced by moving blades 15 in a circular path as hub 17 rotates. As shown in the figures, the left and right sides of aircraft 11 are generally mirror images of each other, having corresponding components on each side of aircraft 11. As described herein, a single reference number may be used to refer to both left and right (as viewed if seated in the aircraft) components when the description applies to both components. Specific reference numbers are used for clarity to refer to specific left or right components when the description is specific to either the left or right component. For example, “rotor 13” may be used in descriptions of both the left rotor and the right rotor, and “rotor 13A” and “rotor 13B” may be used in descriptions that are specific to the left and right rotors, respectively.
The amount of lift produced can be varied by changing the angle of attack, or pitch, of blades 15 or the speed of blades 15, though the speed of rotor 13 is usually controlled by use of a RPM governor to within a narrow range for optimizing performance. Varying the pitch for each blade 15 requires a complex mechanical system, which is typically accomplished using a swashplate assembly (not shown) located on each hub 17.
Each swashplate assembly has two primary roles: (1) under the direction of the collective control, each swashplate assembly changes the pitch of blades 15 on the corresponding rotor 13 simultaneously, which increases or decreases the lift that each rotor 13 supplies to aircraft 11, increasing or decreasing each thrust vector 19 for causing aircraft 11 to gain or lose altitude; and (2) under the direction of the cyclic control, each swashplate assembly changes the angle of blades 15 on the corresponding rotor 13 individually as they move with hub 17, creating a moment in a generally horizontal direction, as indicated by arrows 21, for causing aircraft 11 to move in any direction around a horizontal 360-degree circle, including forward, backward, left and right.
Typically, the collective blade pitch is controlled by a lever that the pilot can move up or down, whereas the cyclic blade pitch is controlled by a control stick that the pilot moves in the direction of desired movement of the aircraft. The collective control raises the entire swashplate assembly as a unit, changing the pitch of blades 15 by the same amount throughout the rotation of hub 17. The cyclic control tilts the swashplate assembly, causing the angle of attack of blades 15 to vary as hub 17 rotates. This has the effect of changing the pitch of blades 15 unevenly depending on where they are in the rotation, causing blades 15 to have a greater angle of attack, and therefore more lift, on one side of the rotation, and a lesser angle of attack, and therefore less lift, on the opposite side of the rotation. The unbalanced lift creates a moment that causes the pitch or roll attitude of aircraft 11 to change, which rotates the thrust vectors and causes aircraft 11 to move longitudinally or laterally.
A tiltrotor aircraft, such as aircraft 11, also has movable nacelles 23 that are mounted to the outer ends of each fixed wing 25. Nacelles 23 can be selectively rotated, as indicated by arrows 27, to any point between a generally vertical orientation, as is shown in FIG. 1, corresponding to a “helicopter mode” for rotor-borne flight using blades 15 to provide lift, and a horizontal orientation, corresponding to an “airplane mode” for forward flight using fixed wings 25 to produce lift. Aircraft 11 may also operate in partial helicopter mode at low speeds, in which rotors 13 and fixed wings 25 both provide part of the required lift for flight. The operation of aircraft 11 typically includes a vertical or short takeoff, a transition from helicopter mode to airplane mode for forward flight, and then a transition back to helicopter mode for a vertical or short landing.
Due to the many variables involved in the control of flight of a tiltrotor aircraft, a computer-controlled flight control system (FCS) 28 automates many of the functions required for safe, efficient operation. FCS 28 actuates flight-control components of aircraft 11 in response to control inputs generated by one or more of the following: (1) an on-board pilot; (2) a pilot located remote from the aircraft, as with an unmanned aerial vehicle (UAV); (3) a partially autonomous system, such as an auto-pilot; and (4) a fully autonomous system, such as in an UAV operating in a fully autonomous manner. FCS 28 is provided with software-implemented flight control methods for generating responses to these control inputs that are appropriate to a particular flight regime.
In the automatic control methods of current tiltrotor aircraft, when a command for a change in longitudinal velocity is received by FCS 28 while aircraft 11 is in full or partial helicopter mode, FCS 28 induces longitudinal acceleration of aircraft 11 by changing the pitch attitude of aircraft 11 to direct thrust vectors 19 forward or rearward. The change of pitch attitude is accomplished by FCS 28 commanding the swashplates to tilt forward or rearward using cyclic control, which causes aircraft 11 to pitch downward in the direction that the aircraft is commanded to fly. For example, when aircraft 11 is commanded by a pilot to fly in the forward direction by moving the cyclic control forward, FCS 28 commands the swashplate for each rotor 13 to tilt forward, and rotors 13 create a forward pitch moment. As shown in FIG. 2, the moment causes the plane of blades 15 to tilt forward and also pitches aircraft 11 in the nose-down direction, which is visible in comparison to ground 29. Thrust vectors 19 are thus rotated toward the forward direction, and the result is movement in the direction shown by arrow 30.
There are several undesirable influences on aircraft 11 using this flight control method, especially in a gusty or windy environment. When the pitch attitude of aircraft 11 is changed due to a command to move in the forward/rearward direction, there is a change in the angle of attack of wings 25 and a corresponding reduction in lift produced by wings 25, and this may produce an undesirable change in the vertical velocity and/or altitude of aircraft 11, which must be countered by changing the vertical climb command. This pitch-attitude-to-vertical-velocity coupling is especially true when hovering or in a low-speed flight condition, and is more pronounced in the presence of a headwind. Using the current automatic flight control method in this situation, aircraft 11 cannot accelerate in the forward direction without a nose-down pitch attitude, and the resulting uncommanded and unwanted vertical motion interferes with the precise vertical control of aircraft 11.
In the automatic control methods of current tiltrotor aircraft, when a command for a change in lateral velocity is received by FCS 28 while the aircraft is in full or partial helicopter mode, FCS 28 induces lateral acceleration of aircraft 11 by changing the roll attitude of aircraft 11 to direct thrust vectors 19 to the left or right. This is accomplished using differential collective blade pitch control, which causes fuselage 23 to tilt right or left in the direction that aircraft 11 is commanded to fly. For example, when aircraft 11 is commanded to fly to the right, FCS 28 commands the collective controls on rotors 13 such that right rotor 13 produces less lift than that being produced by left rotor 13. The resulting thrust imbalance causes aircraft 11 to roll to the right, as shown in FIG. 3, directing thrust vectors 19 to the right and causing aircraft 11 to move in the direction of arrow 31.
This automatic flight control method of tilting aircraft 11 during lateral maneuvering also causes several problems. When aircraft 11 is operating in the area of ground effects, which it must do each time it is in close proximity to a large surface, such as ground 29 during takeoff and landing, the rolling of aircraft 11 will cause one rotor 13 to be closer to ground 29 than the other rotor 13. This difference in relation to ground 29 will cause the ground effects to be greater on one side of aircraft 11 than on the other, which will cause the lift of each rotor 13 to change differently. This difference will cause an additional roll moment on aircraft 11, and this interferes with the precise control of aircraft 11. The rolling of aircraft 11 also tends to blow the air cushion out from under one side of aircraft 11, further degrading the controllability.
When aircraft 11 is moving laterally, or is hovering in a sideward wind, and wings 25 are tilted to the left or right, there is more drag or wind resistance. There is also an increase in down loading, which is the loading of the top of wings 25 by the dynamic pressure caused by rotors 13 and the lateral aircraft velocity. Both of these conditions degrade the controllability in the lateral and vertical axes and require more power than flying level in the same wind conditions.
Aircraft 11 is also subject to upsets from wind gusts, with wind from any direction causing large position displacements when using the current control methods. For example, if aircraft 11 experiences a wind gust from the left side, aircraft 11 will roll to the right. When aircraft 11 rolls to the right, thrust vectors 19 are also rotated to the right, which makes the lateral velocity of aircraft 11 increase to the right. In current tiltrotor aircraft, if FCS 28 is programmed to hold the aircraft over a specified point on the ground, FCS 28 will command aircraft 11 to roll back to the left, causing thrust vectors 19 to oppose the gust and to move aircraft 11 back to the position it occupied before the gust. This method of control has the disadvantage of allowing the gust to displace aircraft 11 a significant distance from its original position before FCS 28 can drive aircraft 11 back to the original position.
Other problems with the current methods of control include high response time to FCS commands and reduced passenger comfort. Response time to forward and lateral velocity commands is high due to the requirement that the attitude of aircraft 11 change for these commands to be executed, and the high inertia of a large, manned tiltrotor, such as aircraft 11, translates into low response frequencies of the system. A significant disadvantage for tiltrotors used to carry passengers is that passenger comfort is compromised by tilting fuselage 23 of aircraft 11 while maneuvering while hovering or in low-speed flight, such as while approaching for a landing and when moving aircraft 11 into position to accelerate to forward flight.
In the automatic control methods of current tiltrotor aircraft, when a command to change the yaw velocity (i.e., the velocity of change of heading) of aircraft 11 is received by FCS 28 while the aircraft is in full or partial helicopter mode, FCS 28 induces a yawing moment using differential longitudinal cyclic control. For example, when aircraft 11 is commanded to yaw to the left, such as when a pilot depresses the left rudder pedal, FCS 28 commands the swashplate for right rotor 13B to tilt forward and commands the swashplate of left rotor 13A to tilt rearward. As shown in FIG. 4, the planes of blades 15A and 15B and the direction of thrust vectors 19A, 19B are tilted in opposite directions, with vector 19A having a rearward thrust component and vector 19B having forward thrust component. Thrust vectors 19A, 19B create a yaw moment, resulting in rotation of aircraft 11 generally about a vertical yaw axis 32 in the direction shown by arrow 33.
While the system and method of the present application is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the application to the particular embodiment disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the process of the present application as defined by the appended claims.